It is known that 2,3-dihydro-6-nitroimidazo[2,1-b]oxazole compounds represented by the following general formula (1), optically active forms thereof and pharmacologically acceptable salts thereof (hereinafter they are simply referred to as oxazole compounds (I)) have an excellent bactericidal effect against acid-fast bacteria (Mycobacterium tuberculosis, multidrug-resistant Mycobacterium tuberculosis and atypical acid-fast bacteria) (JP-A-2004-149527 and WO2005-042542).

wherein R1 represents a hydrogen atom or C1-6 alkyl-group. n represents an integer of 0-6. R2 represents any group of the following general formulas (A)-(G).
The group represented by the general formula (A) is:

wherein R3 represents any group of the following (1)-(6).    (1) a phenoxy group (at least one group selected from the group consisting of a halogen atom, an optionally halogen-substituted C1-6 alkyl group and an optionally halogen-substituted C1-6 alkoxy group may be substituted on the phenyl ring);    (2) a phenyl C1-6 alkoxy group (at least one group selected from the group consisting of a halogen atom, an optionally halogen-substituted C1-6 alkyl group and an optionally halogen-substituted C1-6 alkoxy group may be substituted on the phenyl ring);    (3) a —NR4R5 group, wherein R4 represents a hydrogen atom or C1-6 alkyl group and R5 represents phenyl group (at least one group selected from the group consisting of a halogen atom, an optionally halogen-substituted C1-6 alkyl group and an optionally halogen-substituted C1-6-alkoxy group may be substituted on the phenyl ring);    (4) a phenyl C1-6 alkyl group (at least one group selected from the group consisting of a halogen atom, an optionally halogen-substituted C1-6 alkyl group and an optionally halogen-substituted C1-6 alkoxy group may be substituted on the phenyl rings;    (5) a phenoxy C1-6 alkyl group (at least one group selected from the group consisting of a halogen atom, an optionally halogen-substituted C1-6 alkyl group and an optionally halogen-substituted C1-6 alkoxy group may be substituted on the phenyl ring); and    (6) a benzofuryl C1-6 alkyl group (at least one group selected from the group consisting of a halogen atom, an optionally halogen-substituted C1-6 alkyl group and an optionally halogen-substituted C1-6 alkoxy group may be substituted on the benzofuran ring);the group represented by the general formula (B) is:

wherein R6 represents phenyl group (at least one group selected from the group consisting of a halogen atom, an optionally halogen-substituted C1-6 alkyl group and an optionally halogen-substituted C1-6 alkoxy group may be substituted on the phenyl ring). The group represented by the general formula (C) is:

wherein R7 represents a phenyl C2-10 alkenyl group (at least one group selected from the group consisting of a halogen atom, an optionally halogen-substituted C1-6 alkyl group and an optionally halogen-substituted C1-6 alkoxy group may be substituted on the phenyl ring) or a biphenyl C1-6 alkyl group (at least one group selected from the group consisting of a halogen atom, an optionally halogen-substituted C1-6 alkyl group and an optionally halogen-substituted C1-6 alkoxy group may be substituted on the phenyl ring). The group represented by the general formula (D) is:

wherein R8 represents a phenyl C1-6 alkyl group (at least one group selected from the group consisting of a halogen atom, an optionally halogen-substituted C1-6 alkyl group and an optionally halogen-substituted C1-6 alkoxy group may be substituted on the phenyl ring).
The group represented by the general formula (E) is:

wherein R8 is as described above.
The group represented by the general formula (F) is:

wherein R8 is as described above.
The group represented by the general formula (G) is:

wherein R6 is as described above.
Among acid-fast bacteria, human Mycobacterium tuberculosis is widely known and one third of human being is said to be infected. In addition, Mycobacterium africanum, Mycobacterium bovis and Mycobacterium microti are known as members, of a group of tubercle bacilli together with Mycobacterium tuberculosis and as mycobacteria strongly pathogenic to human beings.
To these tuberculosis, a treatment is performed using those ranked as the primary antituberculous drug, i.e. three drugs including antibacterial drug of rifamycin family (e.g. rifampicin, rifabutin, rifapentine etc.), isoniazid and ethambutol hydrochloride (or streptomycih), or four drugs including those above described plus pyrazinamide.
However, very long period of medication is required for treatment of tuberculosis, which lead to poor compliance resulting in failure of the treatment inconsiderable number of cases. For multidrug resistance of infecting Mycobacterium tuberculosis and tuberculosis caused by multidrug-resistant Mycobacterium tuberculosis, used are secondary antituberculous drugs including kanamycin, enviomycin, capreomycin, paraminosalicylic acid, cycloserine, thioacetazone; quinolone anti-bacterial drugs including ofloxacin, levofloxacin, moxifloxacin, gatifloxacin, sparfloxacin etc. due to their in vitro efficacy; and macrolide antibacterial drugs including clarithromycin and azithromycin, etc. They have, however, strong side effects and a low potency.
Further, the following side effects are reported for the primary antituberculous drugs: hepatopathy, full syndrome, drug allergy, incompatibility with other drugs due to enzymatic induction related to P450 for rifamycin and related anti-bacterial drugs; peripheral neuropathy, induction of severe hepatopathy by concomitant use of rifampicin for isoniazid; decreased vision due to optic nerve disorder for ethambutol hydrochloride; decreased hearing ability by 8th cerebral nerve disorder for streptomycin; hepatopathy, gouty attack associated with increased level of uric acid and vomiting, etc. for pyrazinamide (A Clinician's Guide To Tuberculosis, Michael D. Iseman 2000 by Lippincott Williams & Wilkins, printed in the USA, ISBN 0-7817-1749-3; Kekkaku 2nd edition (1992), by Humiyuki Kuse and Takahide Izumi, Igakushoin; Kekkabku Vol. 74: 77-82 (1999)).
There is a report in fact that cases where standard chemotherapy can not be performed owing to these side effects account for 70% of discontinued medication cases (about 23%, 52 cases) among the total cases (228 inpatient cases in total subjected to investigation) (Kekkaku Vol. 74: 77-82 (1999)).
In particular, it is known that a side effect of hepatic toxicity which is common in rifamycin and related antibacterial drug, isoniazid and pyrazinamide among five drugs used concomitantly on the first line described above develops most frequently. On the other hand, Mycobacterium tuberculosis which shows resistance to antituberculous drugs and that has become multidrug-resistant is increasing, making treatment more difficult.
According to the investigation by WHO (1996-1999), it is announced that among Mycobacterium tuberculosis isolated in the world, the ratio of those which show resistance to any of existing antituberculous drugs reaches 19%, and that of those which show multidrug-resistance is 5.1%. Carriers who are infected with these multidrug-resistant Mycobacterium tuberculosis has assumably reached 60 millions in the world, and further increase of multidrug-resistant Mycobacterium tuberculosis in the future is concerned (April 2001, as a supplement to the journal Tuberculosis, the “Scientific Blueprint for TB drug development”).
Further, it is reported that the mass of death in AIDS patients is caused by tuberculosis and that the number of human beings mixed-infected with tuberculosis and HIV has reached 10.7 millions at the time point of 1997 (Global Alliance for TB drug development). In addition, it is considered that mixed infection involves a higher onset risk of tuberculosis by at least 30 times than otherwise.
Further, bacteria known to be pathogenic to human beings include Mycobacterium avium and Mycobacterium intracellulare which cause recently increasing MAC disease (Mycobacterium avium-intracellulare complex disease) and other atypical acid-fast bacteria such as Mycobacterium kansasil, Mycobacterium marinum, Mycobacterium simiae, Mycobacterium scrofulaceum; Mycobacterium szulgai, Mycobacterium xenopi, Mycobacterium malmoense, Mycobacterium haemophilum, Mycobacterium ulcerans, Mycobacterium shimoidei, Mycobacterium fortuitum, Mycobacterium chelonae, Mycobacterium smegmatis, and Mycobacterium aurum. 
Currently, we are short of promising therapeutic drugs against these atypical acid-fast bacterial diseases, and in the present state, primary antituberculous drugs such as rifamycin and related anti-bacterial drugs, isoniazid, ethambutol, streptomycin and pyrazinamide, and common therapeutic drugs against microbism including quinolone antibacterial drugs or antibiotics are used in combination.